add implicit capabilities to jsvar opend

jsvar.d

@@ -82,6 +82,8 @@
 +/
 module arsd.jsvar;
 
+import arsd.core;
+
 version=new_std_json;
 
 static import std.array;
@@ -545,9 +547,43 @@ private var _op(alias _this, alias this2, string op, T)(T t) if(op != "~") {
 	assert(0);
 }
 
+// a plain `compiles(T(var))` check can be triggered by an implicit cast
+// which we don't want due to endless recursion so this more specific check
+// are better than that anyway
+private template hasVarConstructor(T) {
+	bool helper() {
+		static if(is(T == struct) || is(T == class))
+			static if(__traits(hasMember, T, "__ctor"))
+			foreach(overload; __traits(getOverloads, T, "__ctor"))
+				static if(is(typeof(overload) Params == __parameters))
+					static if(Params.length == 1)
+						static if(is(Params[0] == var))
+							return true;
+		return false;
+	}
+
+	enum bool hasVarConstructor = helper();
+}
+
+unittest {
+	assert(hasVarConstructor!string == false);
+
+	static struct A {
+		this(var a) {}
+	}
+
+	static struct B {
+		this(int a) {}
+	}
+
+	assert(hasVarConstructor!A == true);
+	assert(hasVarConstructor!B == false);
+}
+
 
 ///
 struct var {
+	@implicit
 	public this(T)(T t) {
 		static if(is(T == var))
 			this = t;
@@ -555,6 +591,10 @@ struct var {
 			this.opAssign(t);
 	}
 
+	T opImplicitCast(T)() {
+		return this.get!T;
+	}
+
 	// used by the script interpreter... does a .dup on array, new on class if possible, otherwise copies members.
 	public var _copy_new() {
 		if(payloadType() == Type.Object) {
@@ -990,9 +1030,9 @@ struct var {
 			return this;
 		} else static if(__traits(compiles, T.fromJsVar(var.init))) {
 			return T.fromJsVar(this);
-		} else static if(__traits(compiles, T(this))) {
+		} else static if(hasVarConstructor!T && __traits(compiles, T(this))) {
 			return T(this);
-		} else static if(__traits(compiles, new T(this))) {
+		} else static if(hasVarConstructor!T && __traits(compiles, new T(this))) {
 			return new T(this);
 		} else static if(is(T == Nullable!N, N)) {
 			if(payloadType == Type.Object && this._payload._object is null)